The invention relates to mullite bodies and method of forming mullite bodies.
Porous ceramic bodies have been used in a wide range of applications such as catalyst supports, filtration supports, filtration devices, and high temperature thermal insulation. Porous ceramic bodies are generally made by forming a body from ceramic powders and then heating the body to a temperature sufficient to form a uniform porous monolithic body.
In forming filters using ceramics, typically, a particulate ceramic body is formed. The body is then is heated to temperature to lightly sinter the particles into a uniform monolithic body (i.e., give the body sufficient strength while still having sufficient porosity to allow the useful passage of liquid or a gas). To the lightly sintered body, a thin discriminating layer (i.e., a layer that has a smaller pore size than the lightly sintered body) of a different material is applied to a surface of the body.
For example, a dispersion of colloidal ceramic particles that sinters at a lower temperature is applied to a body already lightly sintered and this coated body is again heated to sinter the colloid particles to form a coating bonded to the lightly sintered body. One sintering step is almost never used due to cracking as a result of the support and the discriminating layer having significantly different shrinkage behavior during sintering (e.g., shrinkage rate and temperature where shrinkage first occurs).
Recently, Moyer et al., describe forming a mullite (ceramic that has a chemistry ranging from about 3Al2O3.SiO2 to 3Al2O3.2SiO2) into a filter (U.S. Pat. Nos. 5,194,154 and 5,198,007). Moyer et al., describe a monolithic filter of large mullite whiskers and mullite filter having a separately applied discriminating layer such as a sintered colloidal alumina coating, a polymeric organic compound or a molecular sieve (e.g., zeolites).
The process of adding a separate discriminating layer to a porous ceramic (e.g., porous mullite) requires added steps (e.g., two or more heating steps). This in turn may increase the possibility of damage due to handling and delamination of the discriminating layer as a result of insufficient bonding and mismatched thermal expansion coefficients. All of this generally leads to an increased cost of the filter and like products.
Accordingly, it would be desirable to provide both a formation method and a ceramic filter and like product that overcomes one or more of the problems of the prior art such as one of those described above.
A first aspect of the present invention is a method for preparing a mullite composition, the method comprising,
a) forming a mixture of one or more precursor compounds having the elements present in mullite,
b) shaping the mixture into a porous green shape
c) applying a nucleation control agent to a portion of the porous green shape,
d) heating the porous green shape of step (c) under an atmosphere and to a temperature sufficient to form a mullite composition comprised substantially of mullite grains that are essentially chemically bound wherein the composition has at least two adjoining regions that have substantially different microstructures.
Surprisingly the present method allows the in situ formation of a thin discriminating layer that has a much smaller pore size than the bulk of the mullite composition. The method surprisingly is also capable of forming mullite compositions that have alternating regions of differing microstructure in one or more directions throughout the composition.
A second aspect of the invention is a mullite composition comprised substantially of mullite grains that are essentially chemically bound wherein the composition has at least two adjoining regions that have substantially different microstructures.
The mullite body of the present invention may be used in any application suitable for mullite. In particular the mullite body may be used in applications requiring a body having two or more regions of differing microstructure. Examples include filters, refractories, thermal and electrical insulators, catalysts and catalyst supports.